High-pressure gas discharge lamp

ABSTRACT

A high-pressure gas discharge lamp (HID [high intensity discharge] lamp) is described which is in particular free from mercury and suitable for use in automobile technology. The lamp is remarkable in particular for a discharge space ( 2 ) which has a volume which is reduced by a given factor in comparison with the volume of the discharge space of known mercury-containing discharge lamps. The quantity of the light-generating substances in the discharge space ( 2 ) is reduced by the same factor in the simplest case, or even more strongly. This avoids the risk of an impairment of the imaging properties of the lamp because of non-evaporated light-generating substances which may shade off a portion of the luminous discharge arc ( 21 ) and/or the tips of the electrodes ( 3 ).

The invention relates to a high-pressure gas discharge lamp (HID [highintensity discharge] lamp) which is in particular free from mercury andsuitable for use in automobile technology.

Conventional high-pressure gas discharge lamps usually contain inaddition to a starter gas on the one hand a discharge gas (for example ametal halide such as sodium iodide or scandium iodide), which representsthe actual light-emitting material (light generator), and on the otherhand mercury, which serves primarily as a voltage gradient former andhas the essential function of promoting the evaporation of thelight-generating substances through a rise in temperature and pressureand of raising the luminous efficacy and burning voltage of the lamp.

Lamps of this kind have come into widespread use because of their goodphotometric properties, and they are increasingly used also inautomobile technology. An additional requirement made in a number ofcases in particular for this application is that the lamps shouldcontain no mercury for environmental reasons.

The problems related to an omission of mercury are essentially that alower operating voltage and accordingly a higher lamp current and alower luminous efficacy are obtained for a given lamp power incontinuous operation, unless measures are provided for at least partlytaking over the functions of the mercury mentioned above.

Thus it is known, for example, from EP 0 581 359 to offset the pinchesof the discharge lamp with respect to the axis of the discharge space ina direction to the lower wall thereof so as to reduce the temperaturedifferences between the wall portions of the discharge space which arein uppermost and lowermost position in the operational state, so that inthis manner the distance between the electrode tips and the lower wallis reduced. This publication, however, relates to a discharge lamp thatcontains mercury.

It was found that such a change is capable of raising the operatingvoltage and luminous efficacy in a mercury-free lamp. This change,however, may also have the result that the non-evaporated substances inthe discharge space, in particular the light-generating saltsaccumulating on the lower wall of the discharge space, adversely affectthe imaging properties of the luminous discharge arc after switching-onof the lamp in that said salts migrate towards the electrode tips andpartly obscure said tips or the luminous discharge arc.

It is accordingly an object of the invention to provide a high-pressuregas discharge lamp with a discharge space whose inner shape has beenchanged (“asymmetrical discharge space”), for example for achieving asubstantially homogeneous temperature distribution in accordance withthe above explanation, wherein the danger of an impairment of theimaging properties by non-evaporated substances in the discharge spaceis at least substantially eliminated.

The invention also has for its object to provide a high-pressure gasdischarge lamp in which the risk of an impairment of the imagingproperties by non-evaporated substances in the discharge space is atleast substantially eliminated in particular in the case in which theelectrodes and the discharge space are mutually asymmetrically arranged(“asymmetrical discharge vessel”), i.e. at least the electrode tips areat a smaller distance from a bottom wall surface of the discharge spacethan from the upper wall thereof (always in the operational position ofthe lamp).

Finally, it is an object of the invention to provide in particular amercury-free high-pressure gas discharge lamp which has an asymmetricaldischarge space and/or an asymmetrical discharge vessel for achieving adesired luminous efficacy and operating voltage, without giving rise tothe risk that substances that are not evaporated in the operationalstate of the lamp partly or wholly obscure the discharge arc or theelectrodes and thus detract from the imaging properties.

According to claim 1, the object is achieved by means of a high-pressuregas discharge lamp with an asymmetrical discharge space and/or anasymmetrical discharge vessel, wherein the discharge space has a volumewhich is reduced by a given first factor in comparison with the volumeof the discharge space of known mercury-containing discharge lamps, andwherein an obscuration of portions of the luminous discharge arc and/orof portions of the electrodes by light-generating substances notevaporated in the operational state is prevented in that the quantity ofthe light-generating substances in the discharge space is reduced by asecond factor which is determined in dependence on the value of thefirst factor and on the distance, defined by the asymmetry, of theelectrodes to a bottom surface that is lowermost in the operationalposition of the lamp.

The starting point here is that the volume of the discharge space of aknown mercury-containing discharge lamp, for example in accordance withU.S. Pat. No. 5,402,037, lies between 20 μl and 50 μl.

It is furthermore assumed that a discharge lamp usually contains a gasfilling in which the light-generating substances are present in an atleast slightly oversaturated quantity, so that also in the operationalstate these substances do not fully enter the gas phase, but a portionthereof remains in solid or liquid form on the bottom of the dischargespace. A reservoir of light-generating substances is thus maintained inthe lamp, with which any losses through diffusion are made up and lamplife is prolonged.

A particular advantage of this solution is that an increase in theluminous efficacy and operating voltage can be achieved while theimaging properties remain the same in discharge lamps containing mercuryand in those that are free from mercury in a simple and reliable manner.

The dependent claims relate to advantageous further embodiments of theinvention.

The embodiment of claim 2 is provided in particular for use inautomobile technology.

The embodiments of claims 3 to 5 relate to preferred embodiments of amercury-free discharge lamp with particularly good imaging properties,while the embodiments of claims 6 and 7 have an enhanced luminousefficacy and operating voltage for a mercury-free gas filling.

Further details, features, and advantages of the invention will becomeapparent from the ensuing description of preferred embodiments, which isgiven with reference to the drawing, in which:

FIG. 1 is a diagrammatic longitudinal sectional view of such anembodiment.

FIG. 1 diagrammatically shows the construction of a first high-pressuregas discharge lamp according to the invention. The lamp of FIG. 1comprises a discharge vessel 1 of quartz glass, which encloses adischarge space 2. The discharge space 2 is bounded inter alia by abottom surface 11, 12 which is in lowermost position in the operationalposition of the lamp and by an upper wall 12 opposite to the former.

The free, first ends of electrodes 3, made from a material of as high aspossible a melting temperature such as, for example, tungsten, extendinto the discharge space 2 from its mutually opposed ends. The secondends of the electrodes 3 are each fastened to an electrically conductingtape or foil 4, in particular a molybdenum foil, through which again anelectrical connection is achieved between connection terminals 5 of thedischarge lamp and the electrodes 3.

To safeguard a vacuumtight entry of the electrodes 3 into the dischargespace 2, the discharge vessel 1 merges into quartz glass portions(pinches or metal-quartz lead-throughs) 6 in the entry locations,wherein the second ends of the electrodes 3, the electrically conductingfoil 2, and portions of the connection terminals 5 are embedded.

An arc discharge 21 (luminous arc) is excited between the tips of theelectrodes 3 when the lamp is in the operational state.

The discharge space 2 is filled with a gas which comprises a dischargegas (light generator) that emits the light radiation through excitationand discharge as well as preferably a voltage gradient former, which mayboth be chosen from the group of metal halides.

The light-generating substances are, for example, sodium iodide and/orscandium iodide, while the voltage gradient formers that may be usedinstead of mercury are, for example, zinc iodide and/or othersubstances, in particular one or several metal halides.

Since the substances used as a replacement for mercury have acomparatively low partial vapor pressure, however, it is necessary tochange the temperature balance in the discharge vessel 1 so as toachieve substantially the same luminous efficacy as with the use ofmercury, i.e. substantially the same luminous flux, as well as anoperating voltage which is as high as possible.

This change in the temperature balance can be achieved with the interiorshape of the discharge space 2 shown in FIG. 1. As is apparent from theFIGURE, the bottom surface 10, 11 in lowermost position in theoperational state (which normally has the lowest temperature in theoperational state of the lamp) has a raised central first portion 10which is surrounded by relatively lowered second portions (“pockets”)11. The first portion 10 has a comparatively small distance to theluminous arc 21 that is formed during operation. This distance shouldpreferably be smaller than the distance between the luminous arc 21 andthe upper wall 12 of the discharge space 2.

This measure at the same time reduces the volume of the discharge spaceby a first factor in comparison with the volume of the discharge spaceof known mercury-containing discharge lamps, which factor is defined bythe shape of the bottom surface 10, 11.

The measure described above achieves that the temperature of thelight-generating substances that have accumulated in the solid state onthe first portion 10 with the lamp being switched off is raised so farthat said substances enter the gaseous state in a sufficient quantityfor achieving a desired, i.e. as high as possible luminous efficacy andburning voltage in continuous operation.

It is possible with the raised first portion 10 of the bottom surface inparticular to achieve a luminous efficacy of the lamp as could hithertobe achieved substantially only with gas fillings containing mercury.Furthermore, the spectral properties and the color point of thegenerated light correspond substantially to those of lamps containingmercury, which is of particular importance for the application inautomobile technology.

The burning voltage of the lamp is also raised thereby in comparisonwith known mercury-free lamps.

In addition, the temperature of the hottest spot of the discharge vessel1, which is usually present on the opposite side, at the upper wall 12,is not raised any further, so that also the maximum thermal load on thelamp is not increased and in particular a lumen maintenance comparableto that of mercury-containing discharge lamps is achieved.

The rise in temperature of only the first portion 10 of the bottomsurface also achieves, finally, that the temperature gradient along thewall of the discharge vessel 1, in particular between the upper andlower sides thereof, is reduced, so that also the thermal stresses inthe vessel are substantially smaller.

It should indeed be safeguarded here that, after switching-on of thelamp, the light-generating substances or other substances not yetevaporated do not cover the electrode tips or the discharge arc 21,including the diffuse region thereof, because the imaging properties ofthe lamp are impaired thereby.

It should furthermore be heeded that the light-generating substancesaccumulated on the bottom surface 10, 11 cannot reach the entrylocations 7 of the electrodes 3, and thus the pinches 6, owing to thetemperature rise occurring during switching-on of the lamp and theresulting migration of these substances, because they could cause damagethrough corrosion or similar effects there in the course of time.

This is achieved according to the invention in that the quantity of thelight-generating substances in the discharge space is reduced by asecond factor which is determined in dependence on the value of thefirst factor mentioned above and on the distance of the electrodes tothe bottom surface 10, 11, in particular the first portion 10 thereof,which distance follows from the asymmetry.

At the same time, however, the quantity of the light-generatingsubstances should remain so large that they are never fully evaporatedalso in the operational state of the lamp (oversaturation), so as tocreate a reservoir in this manner for dealing with diffusion losses andfor prolonging lamp life.

An example will now be given below based on a usual symmetricaldischarge vessel or symmetrical discharge space which has a volume of 27μl and contains 300 μg of light-generating substances.

When the volume of the discharge space is reduced to approximately 18 μlin the case of a mercury-free gas filling, and the first portion 10 inthe embodiment shown in FIG. 1 is raised by approximately 1 mm withrespect to the second portions 11, particularly advantageous imagingproperties are obtained when the quantity of light-generating substancesis reduced to approximately 200 μg. A reduction in luminous efficacy andburning voltage that may arise can be compensated through the additionof rare gas, in particular xenon, in the discharge space 2, i.e. anincrease in the xenon pressure. Experiments have shown that a rise inthe xenon cold pressure of approximately 10 bar to approximately 13 barrenders it possible to raise the luminous efficacy by approximately 5%.It was further found that a rise in the xenon pressure of this order ofmagnitude does not have an appreciable effect on the imaging propertiesof the luminous discharge arc 21.

It is accordingly possible with the invention to realize in particular amercury-free discharge lamp through the reduction of the volume orchange of the discharge space 2 as described above with substantiallythe same luminous efficacy and burning voltage as withmercury-containing discharge lamps, while it is only necessary to reducethe quantity of the light-generating substances in the manner describedso as to safeguard unchanged imaging properties.

The principle of the invention is obviously also applicable tomercury-containing discharge lamps and in general to those dischargelamps in which the volume is not reduced.

1. A high-pressure gas discharge lamp with an asymmetrical dischargespace (2) and/or an asymmetrical discharge vessel (1), wherein thedischarge space (2) has a volume which is reduced by a given firstfactor in comparison with the volume of the discharge space of knownmercury-containing discharge lamps, and wherein an obscuration ofportions of the luminous discharge arc (21) and/or of portions of theelectrodes (3) by light-generating substances not evaporated in theoperational state is prevented in that the quantity of thelight-generating substances in the discharge space (2) is reduced by asecond factor which is determined in dependence on the value of thefirst factor and on the distance, defined by the asymmetry, of theelectrodes (3) to a bottom surface (10, 11) that is lowermost in theoperational position of the lamp.
 2. A high-pressure gas discharge lampas claimed in claim 1, wherein the discharge space (2) does not containmercury.
 3. A high-pressure gas discharge lamp as claimed in claim 1,wherein the volume of the discharge space (2) is approximately 18 μl. 4.A high-pressure gas discharge lamp as claimed in claim 3, wherein thequantity of light-generating substances is approximately 200 μg.
 5. Ahigh-pressure gas discharge lamp as claimed in claim 4, wherein thebottom surface comprises a first portion (10) which is raised byapproximately 1 mm with respect to a surrounding second portion (11). 6.A high-pressure gas discharge lamp as claimed in claim 1, wherein thedischarge space (2) contains a rare gas.
 7. A high-pressure gasdischarge lamp as claimed in claim 6, wherein the rare gas is xenon witha xenon cold pressure of between approximately 8 bar and approximately20 bar, in particular between approximately 10 bar and approximately 15bar.
 8. A lighting unit with a high-pressure gas discharge lamp asclaimed in claim 1.